1. Field of the Invention
The present invention relates to an optical disc and a reproducing apparatus for this optical disc suitable for use, for example, in a CD ROM (compact disc read only memory) or the like.
2. Description of the Prior Art
The assignee of the present application has previously proposed an optical disc and a reproducing apparatus for reproducing such optical disc as shown in FIGS. 1 and 2 (see Japanese Patent Application No. 1-22212). More specifically, in FIG. 1, reference numeral 1 generally designates a disc reproducing apparatus, wherein an optical disc 2, on which multiple-value information signal composed of four digital signal values is recorded in a groove, is driven by a spindle motor 3 and controlled so as to rotate at a predetermined rotational speed about a central axis 0.
Actually, on this optical disc 2, there are formed recording tracks. Each of the tracks is formed of a groove 4. The groove 4 is, as shown in FIG. 2, is divided into a data recording area AR.sub.DT and a reference signal recording area AR.sub.REF at predetermined time intervals, and the recording tracks formed of grooves 4 are concentrically formed with a track pitch represented by Tp.
The groove 4 is formed so as to have a depth of .lambda./8 with respect to a wavelength .lambda. of a laser beam L.sub.1 irradiating the optical disc 2, and a groove width Tw having a value Tp/2 with respect to the track pitch Tp.
Further, the groove 4 in the data recording area AR.sub.DT has a first condition S.sub.1 where the groove center GC is formed with an offset of 1.5.DELTA.A on the upward side from the track center TC, a second condition S.sub.2 formed with an offset of 0.5.DELTA.A on the upward side, a third condition S.sub.3 formed with an offset of 0.5.DELTA.A on the downward side from the track center TC, and a fourth condition S.sub.4 with an offset of 1.5.DELTA.A on the downward side, which are repeated respectively corresponding to values of a four-value digital signal [3].sub.4, [2].sub.4, [1].sub.4 and [0].sub.4, for example, in a clock cycle of this digital signal.
On the other hand, the reference signal recording area AR.sub.REF is divided into a synchronizing signal recording area AR.sub.SYS, and an amplitude signal recording area AR.sub.AD, wherein a synchronizing signal and a reference signal are respectively recorded therein.
More specifically, in the synchronizing signal recording area AR.sub.SYS, pits P are formed on the track center TC by periodically interrupting the groove 4 at a rate equal to the clock frequency of the digital information signal. When pits P are reproduced, they generate a synchronizing signal having a frequency equal to the clock frequency of the digital information signal.
Also, in the subsequent amplitude signal recording area AR.sub.AD offset information related to the groove 4 formed on the data recording area AR.sub.DT is recorded.
More specifically, the groove 4 formed on the data recording area AR.sub.DT includes the first condition S.sub.1 with the largest offset on the upward side from the track center TC and the fourth condition S.sub.4 with the largest offset on the downward side which are respectively formed, for example, for five cycles of the system clock.
Incidentally, in the optical disc reproducing apparatus 1, the laser beam L.sub.1 having a wavelength .lambda. emitted from a laser light source 5 is converted to a parallel light by a collimator lens 6, passed through a beam splitter 7, and then converged on the optical disc 2 through an objective lens 8, thereby forming a beam spot SP.sub.PB for reproduction.
A reflected laser beam L.sub.2 which is composed of the reproducing beam spot SP.sub.PB reflected on the optical disc 2, passes through objective lens 8, is reflected by the beam splitter 7 by 90 degrees and is converged by condenser to a photo detector 10 which is split into two parts 10A and 10B in the recording track direction.
Received light output signals S.sub.PD1 and S.sub.PD2 generated from respective light receiving elements 10A and 10B of the split photo detector 10 are inputted to a subtractor circuit 11 and an adder circuit 12 respectively, these circuits being composed of operational amplifier circuits.
The subtractor circuit 11 calculates the difference between respective received light output signals S.sub.PD1 and S.sub.PD2 from the split photo detector 10, and delivers the resulting difference signal Spp to a multiple-value signal reproducing circuit 13.
Also, the adder circuit 12 calculates the sum of respective received light output signals S.sub.PD1 and S.sub.PD2 and delivers the resulting sum signal S.sub.RF to a clock signal generating circuit 14, circuit 14 having a phase-lock loop (PLL) configuration.
With this arrangement, the clock signal generating circuit 14 generates a clock signal S.sub.CK based on the synchronizing signal recorded on the synchronizing signal recording area AR.sub.SYS in the reference signal recording area AR.sub.REF, and delivers the same to the multiple-value signal reproducing circuit 13.
The multiple-value signal reproducing circuit 13, using the offset information in the groove 4 recorded on the amplitude signal recording area AR.sub.AD in the reference signal recording area AR.sub.REF, sets the levels of the difference signal Spp corresponding to the four values of the digital information signal [3].sub.4, [2].sub.4, [1].sub.4 and [0].sub.4, and samples the difference signal Spp at the timing of the clock signal S.sub.CK, to reproduce the four values of the digital information signal DT recorded on the data area AR.sub.DT. This four-value digital information signal DT is outputted to the outside through an output terminal 15.
Incidentally, when an information signal is recorded by displacing the groove 4 in the direction perpendicular to the direction of the groove, if the width Tw of the groove 4 is changed with respect to the track pitch Tp, the level of the difference signal Spp is largest when the groove width Tw is equal to half the track pitch Tp or Tp/2, whereas the level of the sum signal SRF is substantially zero.
Therefore, in the above-mentioned example, by setting the width Tw of the groove 4 to the value Tp/2, when the reproducing beam spot SP.sub.PB scans the data recording area AR.sub.DT and the amplitude signal recording area AR.sub.AD, the level of the difference signal Spp changes in proportion to a displacement amount of the groove 4 in the direction perpendicular to the direction of the groove while the level of the sum signal S.sub.RF maintains a substantially constant value.
On the contrary, when the reproducing beam spot SP.sub.PB scans the synchronizing signal recording area AR.sub.SYS, and the groove 4 is intermittently formed on the track center TC, the level of the difference signal Spp maintains a zero level while the level of the sum signal S.sub.RF changes in accordance with the interruptions in the groove 4.
Thus, the synchronizing signal is recorded on the synchronizing signal recording area AR.sub.SYS by interrupting the groove 4, while the multiple-value signal and the reference signal are recorded in the data recording area AR.sub.DT and the amplitude signal recording area AR.sub.AD in the form of displacement amounts of the groove 4 in the direction perpendicular to the direction of the groove, whereby the multiple-value signal and the reference signal are only derived as the difference signal Spp through the subtractor circuit 11, and the synchronizing signal is only derived as the sum signal S.sub.RF through the adder circuit 12. This makes it possible to separate the synchronizing signal from the multiple-value signal and the reference signal and securely and easily detect the same.
Also, in this example, as shown in FIG. 3, the data recording area AR.sub.DT in the groove 4 (FIG. 3A) on the optical disc 2 is displaced in the direction perpendicular to the direction of the groove in the first, second, third and fourth conditions S1, S2, S3 and S4 corresponding to the values [3].sub.4, [2].sub.4, [1].sub.4 and [0].sub.4 of the four-value digital information signal over every groove length LG corresponding to, for example, three cycles of the clock signal S.sub.CK (FIG. 3B).
The spot diameter L.sub.SP of the reproducing beam spot SP.sub.PB formed on the optical disk 2 by the laser beam L.sub.1 emitted from the laser light source 5 is set to a sufficiently small value with respect to the groove length LG of the groove 4.
Thus, in the multiple-value signal reproducing circuit 13 in the optical disc reproducing apparatus 1 (refer to FIG. 1), the inputted push-pull signal Spp (FIG. 3C) is sampled at times t.sub.1, t.sub.2, t.sub.3, t.sub.4, t.sub.5, . . . , i.e., at the rising edge of the second clock cycle which corresponds to the substantially central portion of each groove length LG. The output of multiple value reproducing circuit 13 is held for the subsequent three clock cycles, whereby the data recording area AR.sub.DT can be correctly reproduced to derive the four-value digital information signal DT (FIG. 3D).
Thus, conventionally, on the basis of a four-value digital signal, a groove displaced at every predetermined groove length is irradiated with a reproducing beam spot having a spot diameter shorter than the groove length, and the resulting differential signal is sampled to detect displacement amounts of the groove, to thereby detect the four-value digital signal.
As is apparent from the foregoing, conventionally, to derive the four-value digital information signal (multiple-value signal), the groove 4 must be formed on the optical disc 2 such that the groove center cc is offset by 1.5.DELTA.A and 0.5.DELTA.A from the track center TC on the upward side and by 1.5.DELTA.A and 0.5.DELTA.A from the track center TC on the downward side. This complicates forming the groove 4 on the optical disc 2 and complicates the signal processing in the optical disc reproducing apparatus for deriving the four-value digital information signal therefrom.